1. Field of the Invention
The present invention relates to a printed circuit board on which electronic parts with leads are mounted and their soldering surfaces are coated with a coating material, and an electronic part having a plurality of leads protruding from its case body. More particularly, this invention relates to a printed circuit board with electronic parts mounted thereon, whose leads and soldered portions are buried in a coating material to prevent the occurrence of a leak between the leads and the solder lands, and an electronic part designed to suppress the occurrence of cracks in a coating layer formed by coating a coating material on the electronic part and a printed board after the electronic part is mounted thereon.
2. Description of the Related Art
It is necessary to provide waterproofing and dampproofing measures to electronic parts which are used under extreme environmental changes, such as electrical equipments for a vehicle.
FIGS. 12 through 14 illustrate a conventional waterproof and dampproof printed circuit board m. Electronic parts a are mounted on a printed board g by inserting their leads f through the land holes (not shown) formed in the printed board g and then soldering those portions of the leads f which protrude from the soldering surface side of the printed board g to this printed board g.
Then, a coating material i is coated on the outer surfaces of the electronic parts a and the printed board g by known means, thereby making the printed circuit board m waterproof and dampproof.
According to this conventional printed circuit board m, however, the coating material is not easily coated on the free end portions of the leads f protruding from the soldered portions and on the soldered portions themselves, and the coating layer, if the coating is successful, will not be thick enough so that those portions are likely to be exposed through the coating layer i. Water w may stick on any exposed portion due to dew condensation, causing a leak (see FIG. 14).
FIGS. 22 and 23 illustrate a conventional electronic part mounted on a printed board, the electronic part and the printed board coated with a coating material.
Referring to those diagrams, an electronic part a comprises a case body b of a rectangular parallelopiped with one side open, a hybrid IC d, which has predetermined electronic elements c formed on a ceramic substrate or a glass substrate and is retained in the case body b, and a resin material e, which is injected into the case body b to bury the hybrid IC d.
In these diagrams, leads (only one shown) denoted by "f" extend from the hybrid IC d and protrude out of the case body b.
After this electronic part a is mounted on a printed board g with its mount surface h contacting this printed board g, the electronic part a and printed board g are coated with a thermoplastic coating material i.
According to the conventional electronic part a, however, the side walls of the case body b stand nearly perpendicular to the printed board g at the peripheral portion of that area where the case body b contacts the printed board g. Before the coating material i, coated on the electronic part a, becomes hardened, the coating material i may move downward and become hardened. If this happens, a relatively thin coating layer i'-a (portion A in FIG. 23) is formed on the side walls j of the case body b and a relatively thick coating layer i'-b (portion B in FIG. 23) is formed on the printed board g. In addition, the thickness of the coating layer drastically changes at the boundary between those coating layers i'-a and i'-b of different thicknesses (portion C in FIG. 23).
If there is a sharp change in the thickness of the coating layer i', stress would be concentrated on that portion C due to thermal contraction after the coating material i is hardened, causing a crack there. In addition, moisture will enter through the crack in the coating layer i', which may result in the malfunction of the circuit.